The brain can have extremely powerful effects on a person's body. Under certain circumstances, medical patients have been known to alter their own blood pressure, lower their body temperature and even manifest a tumor in response to psychological stimuli. The Clown, an artificial being which depended on input from living brains, used this to his advantage in keeping several members of Viorsa's species, and later Harry Kim and B'Elanna Torres, captive. When an individual tried to leave the Clown's artificial environment, he would begin to raise their noradrenaline levels by pretending to cut off their heads with his guillotine. With enough stress, he could induce a heart attack and literally scare the individuals to death. (DS9: "Things Past"; VOY: "The Thaw")